Alloy steel



. bides can exist within the range of concentration Patented May '21,1940 UNITED STATES PATENT OFFICE ALLOY STEEL Karl Torkel Berglund,Sandviken, Sweden, as-

signor to Sandvikens Jcrnverks Aktiebolag, Sandviken, Sweden, a companyof Sweden No Drawing. Application April 28, 1938, Serial No. 204,754. InSweden May 3, 1937 2 Claims.

loy base the composition of which lies within the following approximatelimits:

' Per cent Carbon 0. 20- 2. 50 Chromium 8 -22 Iron, maximum about 90Having regard to the resistancy to corrosion the lower limit of thechromium content of all of these steels maybe placed at about 2%- Withless exacting requirements as to resistancy'to corrosion, somewhat lowerchromium contents may also be used. The upper limit of the chromiumcontent is at about 20%, having regard to the hardenability of thesteels.

The carbon content that is most suitable for difierent fields of use isdetermined, in the main, by the maximum hardness that it is sought toattain by hardening, and also by the desired qualities of the edge ofthe tool. The maximum hardness that it is possible to obtain byhardening, increases with increasing carbon content, but simultaneously,the difficulties connected with shaping and working the steel in hot andcold state also increase.

If the steel is to be used for articles having a thin cutting edge of asuificient durability for practical purposes, a carbon content of atleast 0.50%, and preferably not lower than 0.70%, is required. Anincreased carbon content gives as a rule a better edge, but havingregard to the possibility of working the steel by cold rolling and colddrawing and to the difficulties of rendering the carbides in asufficiently finely divided state, a higher carbon content than about1.35% can hardly be considered for this purpose. The improved edgeobtained through an increased carbon content is explained by theincreased quantity of hard and wear-resistant carbides. An increasedcarbide content, however, impairs as a rule the resistancy tocorrosionof the steel, due to the chromium content of the carbides beingso high that the chromium content of the ground mass is reduced.

According to existing equilibrium diagrams of the alloy systemiron-chromium-carbon, two carhere contemplated, namely, a trigonalcarbide (Cr,Fe)1Cs and a cubic carbide (Cr,Fe) 4C or, probably morecorrectly, (Cr,Fe)2aCs. At temperatures above about 800 C. only thetrigonal carbide can exist when equilibrium is attained, whereas attemperatures below about 800 C. also the cubic carbide appears togetherwith the trigonal one.

Generically speaking the conclusion may be drawn that for one and. thesame carbon content the amount by weight of carbides when in the cubicphase should be 1.5 to 1.8 times the amount obtained if the carbides arein the trigonal phase.

On the other hand, owing to its greater quantity the cubic carbide mayoccasion that, for a certain chromium content of the steel, the groundmass becomes poorer in chromium, with the resultant risk of inferiorresistancy to corrosion.

According to the present invention the carbides or at least the greaterportion of the same may be obtained in the cubic phase by incorporatingwith a chromium steel containing 0.20 to 1.35% carbon and 8 to 22%chromium, a further alloy component of molybdenum, cobalt and copper.The steel according to the present invention is thus mainlycharacterized by that, in addition to an alloy base of the compositionabove set forth, and normal amounts of manganese, silicon, phosphorus,and sulphur present in commercial iron and steel, it also contains thecombined alloy component molybdenum-cobalt-copper, the content of theseelements being each at least 0.20% and together at least 0.60% and atthe most of the weight of the steel thus alloyed. Molybdenum may bewholly or partially replaced by tungsten.

It has been proved by tests that the resistancy to corrosion of an alloysteel according to'this invention is not impaired but, on the contrary,is improved both in the annealed state and, above all, in the hardenedstate.

This twofold effect of a combined alloy component of molybdenum, cobaltand copper was not to be expected in view of the fact that a moderateaddition of one of these elements alone does not result in anynoticeable deviation from the equilibrium diagram foriron-chromium-carbon alone.

In addition to the gain of an improved resistancy to corrosion due tothe said combined addition of alloy components, the advantage of alarger quantity of carbides for a. given carbon content is also gained,and thus greater hardness and increased wearing strength in the hardenedstate, and it has also been found that the complex cubic carbide that isformed under these conditions, can more easily be obtained in a finelydispersed state in connection with the shapmosphere of carbon dioxide soas to eliminate the risk of the carbides being oxidized by the oxygen ofthe air. A great number of tests have ven the result that in the case ofa chromium steel containing about 1% carbon and from 13 to 14% chromiumbut having no other intentional alloy components, the quantity of thecarbides amount to from 7 to 8 per cent ofthe weight of the steel.

A further addition of moderate quantities of manganese, cobalt or copperdoes not noticeably change the quantity of the carbides. On the otherhand, a certain increase of the quantity of carbides is obtained by anaddition of tungsten or molybdenum. If, however, in accordance with thepresent invention, a composite addition of molybdenum, cobalt and copperis made, for instance, about 1% of each substance, the carbide contentof the steel is increased to 18 or 19%. This increase of the carbidecontent is quite surprising in view of the fact that cobalt and coppereach by itself does notincrease the carbide content.

A roentgenographic examination of carbide residues has given thefollowing results: In a chromium steel of this type without extra alloycom- .ponents the carbides consisted of approximately equal partstrigonal carbide and cubic carbide, and thus conformed with therequirements of the equilibrium diagram. On the other hand, the carbideresidue of a chromium steel to which the composite componentmolybdenum-cobalt-copper had been added, proved to consist practicallycompletely of cubic carbide, in spite of the fact that the said steelhad been heated during a long period of time at about 860 C. that is tosay, at a temperature range at which, accordinging to availableequilibrium diagrams, the trigonal carbide only should be present.

The great resistancy to corrosion shown by a steel of the said typecontaining molybdenumcobalt-copper will be explained in part, it isbelieved, by the following analyses shown by the carbide residues:

Percent Percent Percent Fe Cr Cal-hides of a chromium steel withMo-Co-Gu-addition. 5. 64 40. 7 47. 3 carbides of a chromium steelwithout extra alloy elements 8. 20 26. 63. 4

Per cent Carbon 1.02 Manganese 0.41 Silicon 0.19 Chromium 12.9Molybdenum 0.95 Cobalt 1.49 Copper 1.00

The analysis of the carbide residue of this steel corresponds to theformula (Cr, Fe, Mn, Mo, C0)22.44Cs.oo

a value which, having regard to the possible accuracy of analysis,corresponds exceedingly well to the theoretical formula (Cr, Fe) 2308,and whichis also in keeping with the results of the roentgenographieexamination.

It may be pointed out that the addition of copper appears to be ofessential importance to the marked increase in resistancy to corrosionas well as to the formation of the desired cubic carbide.

As regards the manganese content it may be mentioned that in a steelaccording to the present invention the manganese content should be lessthan 0.65%, since with the composite alloy component here contemplated agreater optimum hardness can be obtained below this limit than at ahigher manganese content.

An alloy steel according to the present invention which is particularlysuitable for the manufacture of fine hardened edge tools, such as razorblades and surgical instruments, contains:

For special fields of use it may be desirable slightly to modify themechanical and other properties of the alloy steels by an addition offurther alloy elements, over and above the composite alloy componentmolybdenum-cobalt-copper. Thus, the steel according to the presentinvention may contain percentages exceeding 0.05% of one or more of thealloy elements: Beryllium, boron, aluminium, titanium, vanadium,arsenic, selenium, zirconium, niobium, tin, antimony, tantalum, uranium.

Since, as previously stated, molybdenum may be replaced wholly orpartially by tungsten, the term molybdenum in the following claimsshould be construed as covering tungsten also.

I claim:

1. An alloy steel of the martensitic type, which is stainless in thehardened condition, characterized by containing when in hardenedcondition,

substantially all of the carbides in the cubic phase and possessinggreat hardness and wearing strength, comprising as its principalalloying ingredients carbon in the proportion of 0.20 to 1.35%, chromiumin the proportion of to 22%, nickel less than 0.20%, phosphorus lessthan 0.05%, molybdenum in the proportion of 0.20 to 5%, cobalt in theproportion'of 0.20 to 5%, copper in the proportion of 0.20 to 5%, theremainder being iron with impurities in an amount which will not alterthe properties of the alloy.

2. An alloy steel of the martensitic type, which is stainless in thehardened condition, charac-- terized by containing when in hardenedcondition substantially all of the carbides in the cubic phase andpossessing great hardness and wearing strength, comprising as itsprincipal alloying ingredients carbon in the proportion of 0.70 to1.35%, chromium in the proportion of 10 to 18%, nickel less than 0.20%,phosphorus less than 0.05%, molybdenum in the proportion of 0.20 to 2%,cobalt in the proportion of 0.20 to 2%, copper in the proportion of 0.20to 2%, the remainder being iron with impurities in an amount which willnot alter the properties of the alloy.

KARL TORKEL BERGLUND.

